A polyarylene sulfide, especially polyphenylene sulfide is known as an engineering plastic which is excellent in mechanical strength, heat resistance and the like and which has good electrical characteristics and high rigidity. Thus it is widely employed as a variety of materials such as electronic machinery parts and electrical machinery parts.
As a process for producing a polyarylene sulfide, there has heretofore been employed in general, a process in which a dihalogenated aromatic compound such as p-dichlorobenzene and a sodium salt such as sodium sulfide are reacted with each other in the presence of an aprotic organic solvent such as N-methyl-2-pyrrolidone (hereinafter sometimes abbreviated to “NMP”) to effect a polymerization condensation reaction.
In the above-mentioned process however, a sodium halogenide as a by-product, which is insoluble in a solvent such as NMP, is incorporated in a resin, whereby the removal of the by-product by cleaning has been far from easy. Moreover, it has been extremely difficult to continuously treat at a high temperature, a polymer and sodium halogenide as a by-product that are formed in the aforesaid process.
Under such circumstances, it has been found that continuous treatment of the polymer at a high temperature is made possible by a method in which the polymerization condensation reaction is conducted by using a lithium salt in place of the sodium salt in the presence of NMP as a solvent to form a lithium halogenide, since the lithium halogenide is soluble in a number of aprotic organic solvents such as NMP as a polymerization solvent, thus enabling it comparatively easy to lower the concentration of lithium in the resin (refer to Japanese Patent Application Laid-Open No. 207027/1995 (Heisei 7).
In the above-mentioned process however, lithium sulfide as a starting raw material for polymerization reaction is obtained, for instance, by reacting lithium chloride with sodium hydroxide to obtain lithium hydroxide, reacting the resultant lithium hydroxide with hydrogen sulfide to obtain lithium hydrosulfide, and subjecting the resultant lithium hydrosulfide to hydrogen sulfide removing reaction. In such reaction system, when lithium chloride is reacted with sodium hydroxide, sodium chloride is formed as a reaction by-product. The resultant sodium chloride, when being subjected to polymerization reaction as such, gives rise to a problem same as the problem arising from the foregoing sodium salt. Accordingly, it is customary that slurry solution containing lithium hydrosulfide thus formed is subjected to solid-liquid separation to separate it into a solvent portion containing lithium hydrosulfide and solid matter containing a by-product salt such as sodium chloride, and the solvent portion is subjected to next lithium sulfide synthesis step, while the by-product salt such as sodium chloride is subjected to drying treatment and is thrown into the discard.
In an ordinary synthesis system, sodium chloride is readily dissolved in water and can be discharged outside the system as waste water. However, in the case of recycling the solvent phase or the like which is obtained by solid-liquid separation of PAS polymerization reaction mixture to use as a starting material, there exists a small amount of an oligomer in the solid mixture. In this case, there is caused a problem in that the by-product salt such as sodium chloride which is formed in the aforesaid reaction and subjected to drying treatment is markedly inferior in solubility in water, thus takes a long time to be dissolved in water and besides, the SS and COD in water after dissolution are made high, thereby inevitably increasing the waste water treatment expense.
It is thought that the surface of sodium chloride particles are covered by the PAS oligomer contained in the starting liquid material owing to the drying treatment, and thus the solubility of such sodium chloride in water is worsened, and that in the case where the dryingly treated matter is dissolved in water to form waste water, the SS and COD are unreasonably increased by the PAS oligomer, residual solvent, etc. in water.